1. Field of the Invention
This invention relates to a novel method of hardening steel, and more particularly, to a hardening method which can provide machinery parts or the like with a sufficient hardness as required thereof to minimize the strain developed therein.
2. Description of the Prior Art
Various methods are known for hardening the surface layer of steel to prevent the development of strain therein. Induction hardening is applicable to only a limited range of steel materials, and not suitable for hardening materials having complicated shapes. Hardening by carburization requires only a relatively short time, since penetration of carbon is effected by heating at a higher temperature of, say, 800.degree. C. to 950.degree. C., than the A.sub.1 transformation point. According to this method, however, it is difficult to obtain an accurate case depth, particularly when a thin carburized layer is desired. The internal structure of the steel to be treated undergoes transformation from austenite to martensite, and a sudden volumetric change takes place, so that strain develops to a significant extent. Another method known as nitriding, in which steel is treated at a lower temperature, i.e., 500.degree. C. to 570.degree. C., does not bring about any phase change in the steel, or any heavy strain therein. But as the hardness thereby obtained is attributed to the formation of a nitride and a diffusion layer, the surface treated by this method obtains only a thin hardened layer, and fails to have a sufficiently thick hardened layer. In order to increase the degree of hardening in the surface layer, it is necessary to add elements having a high affinity for nitrogen, for example, Al, W, Mo, Cr and V.